The invention concerns an actively shielded superconducting magnet configuration for generating a homogeneous magnetic field B0 in a volume under investigation,                with a radially inner superconducting main field coil which is disposed rotationally symmetrically about a z-axis,        and with a coaxial radially outer superconducting shielding coil which is driven in the opposite direction,        wherein the magnet configuration consists of the main field coil, the shielding coil and a ferromagnetic field-shaping device,wherein the ferromagnetic field-shaping device is disposed radially inside the main field coil,        the main field coil consists of an unstructured solenoid coil or of several radially nested unstructured solenoid coils which are operated in the same direction, and        the extension Labs of the shielding coil in the axial direction is smaller than the extension Lhaupt of the main field coil in the axial direction.        
A magnet configuration of this type is disclosed in DE 2008 020 107 A1, published after the priority date of the instant invention.
Nuclear magnetic resonance (NMR) is a method of investigating the properties of a sample. NMR spectroscopy is used to analyze the chemical composition (or the chemical bonds) of a sample. NMR tomography is generally used to determine the proton density (or the water content) as a function of the location in a relatively large sample (e.g. part of a human body) in order to obtain information about the internal structure of the sample. In both cases, the basic principle of NMR consists in irradiating RF (radio frequency) pulses into a sample located in a static magnetic field and measuring the RF reaction of the sample. The RF reaction gives information about the properties of the sample. Particularly strong homogeneous static magnetic fields are generally preferred for NMR, since they produce the measuring results with the best quality.
Superconducting magnet coils can generate high magnetic field strengths, wherein the superconducting magnet coils are generally cooled with liquid helium in a cryostat to a typical operating temperature of 4.2 K. Solenoid-shaped magnet coils which surround a circular cylindrical or spherical or ellipsoidal volume under investigation are thereby particularly often used.
In order to homogenize the static magnetic field in the volume under investigation (“shimming”), ferromagnetic material is conventionally disposed in the vicinity of the volume under investigation, in particular, inside the main field coil (“passive shim”) cf. e.g. U.S. Pat. No. 6,897,750, and also additional magnetic field coils (shim coils) are conventionally provided, the magnetic field of which is superimposed on the magnetic field of a main field coil (“active shim”). U.S. Pat. No. 6,265,960 also discloses superconducting shim coil systems in the cryostat. Both the active and the passive shim systems are based on the fact that the main field coil and the shim system together produce a homogeneous magnetic field in the volume under investigation.
Unless particular measures are taken, a strong magnetic field in the volume under investigation is accompanied by a noticeable ambient magnetic field. This ambient magnetic field is also called stray field and is basically not desired, since it can destroy technical devices in the surroundings. Stray fields can e.g. delete magnetic memories of hard discs or credit cards or cause failure of pacemakers. Stray fields are reduced, in particular, by providing a shielding coil radially outside of the main field coil, which generates a magnetic dipole moment which is substantially of the same magnitude but of opposite direction compared to the main field coil.
The main field coil e.g. according to prior art cited in EP 1 564 561 A1, FIG. 1, comprises several windings of superconducting wire, which are disposed axially next to one another, and thereby represents a structured solenoid coil. The use of a structured solenoid coil as main field coil is advantageous, since the spatial dependence of the magnetic field in the volume under investigation can be designed relatively easily through the type of structure such that in total, i.e. together with the magnetic field, which is generated by the shielding coil, one obtains a homogeneous magnetic field in the volume under investigation. The influence of the shielding coil on the homogeneity of the magnetic field in the volume under investigation is thereby generally relatively small due to the larger radial separation from the volume under investigation compared to the main field coil. The windings of this structured solenoid coil are basically held by a mechanical holding device and are generally located in the winding chambers of a coil body. Due to the magnetic field generated by the windings, the windings strongly attract each other, wherein they are forced in an axial direction against the holding device, in general, the lateral limiting surfaces of the winding chambers. In particular, in magnet configurations for generating particularly strong magnetic fields of e.g. 6 T and more, the associated surface pressure can reach very high values.
One essential disadvantage of such magnet configurations with structured main field coils consists in that these very high surface pressures can cause mechanical relaxation processes in the bordering windings of superconducting wire, wherein these become normally conducting and trigger a so-called quench due to their negligibly small thermal capacitance at the low operating temperature. This result is undesirable and expensive, since the magnet coil heats up from the operating temperature to values in the region of 40 to 80 K during a quench, the expensive liquid helium, which is used for cooling, evaporates and is lost, and restarting of the magnet configuration may be associated with time delays of several days.
U.S. Pat. No. 6,617,853 discloses a magnet configuration with a main field coil with structured and unstructured solenoid coils. According to this document, it is possible to simplify magnet configurations by disposing a field-shaping device of magnetic material radially inside the main field coil. However, the main field coils according to U.S. Pat. No. 6,617,853 comprise at least partially structured solenoid coils to ensure generation of sufficiently homogeneous magnetic fields. In accordance with U.S. Pat. No. 6,617,853, simplified main field coils with field-shaping devices of magnetic material can be realized only when the field-shaping device at least partially has a shorter radial separation of less than 80 mm from the magnet axis and therefore a sufficiently high efficiency. Magnet configurations with a larger useful diameter of e.g. 30 cm and more cannot be realized with this restriction.
Actively shielded magnet configurations in accordance with US 2006/0061361 manage completely without any sections with structured solenoid coils as main field coil. These configurations also comprise field-shaping devices of magnetic material radially inside the main field coil, however, without the restriction of the small radial separation of the field-shaping device from the magnet axis. The structure of the main field coil completely without structured solenoid coils becomes possible by using a suitably dimensioned magnet body of magnetic material radially outside of the main field coil. However, for magnet configurations with a useful diameter of e.g. 60 cm and more, the magnetic body and therefore the entire magnet configuration would become very heavy, which would increase the transport cost and limit the possibilities of installing the magnet configuration due to the large floor loading.
EP 0 332 176 A2 discloses magnet configurations which comprise a main field coil, a shielding coil, a field-shaping device of magnetic material, which is disposed radially inside the main field coil, and a yoke magnetic shielding of magnetic material, which is disposed radially outside the shielding coil, wherein the axial extension of the shielding coil is larger at that location than the axial lengths of the main field coil and the yoke magnetic shielding. This prior art document proposes designing the main field coil as a structured solenoid coil. In accordance with the teaching of EP 0 332 176 A2, a magnet configuration with a sufficiently homogeneous magnetic field B0 in the volume under investigation cannot be realized with an unstructured solenoid coil as main field coil, but only with a structured solenoid coil as the main field coil.
DE 10 2008 020 107 A1 (published after the priority date of the instant invention) discloses an actively shielded magnet configuration consisting of a radially inner main field coil, a radially outer shielding coil as well as a ferromagnetic field forming device disposed within the main field coil. The main field coil consists of an unstructured solenoid coil or of a plurality of radially nested unstructured solenoid coils driven in the same direction, wherein the shielding coil is axially shorter than the main filed coil. The magnetic filed produced along the z-axis by the main filed coil and the shielding coil has a field strength minimum at the center and maxima at each side of the center. The ferromagnetic field forming device produces a field shape having a maximum in field strength in the center as well as minima in field strength at each side of the center.
It is therefore the underlying purpose of the invention to provide an actively shielded superconducting magnet configuration with a homogeneous and particularly high magnetic field B0 in the volume under investigation, the structure of which is considerably simplified, in particular, wherein the main field coil can exclusively be constructed from unstructured solenoid coils and the overall magnet configuration can be designed to be much more compact.